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Article

Laser-Induced Tar-Mediated Sintering of Metals and Refractory Carbides in AirXining Zang, Kiera Y. Tai, Cuiying Jian, Wan Shou,

Wojciech Matusik, Nicola Ferralis, and Jeffrey C. GrossmanACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.0c04295 • Publication Date (Web): 03 Aug 2020

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1

Laser-Induced Tar-Mediated Sintering of Metals and

Refractory Carbides in Air

Xining Zang,1 Kiera Y. Tai,1 Cuiying Jian,1,2 Wan Shou,3 Wojciech Matusik,3 Nicola Ferralis,1

Jeffrey C. Grossman1*

1 Department of Materials Science and Engineering, Massachusetts Institute of Technology,

Cambridge, MA, 02139, USA

2 Department of Mechanical Engineering, York University, 4700, Keele Street, Toronto, ON,

M3J 1P3, Canada

3 Computer Science and Artificial Intelligence Laboratory, Massachusetts Institute of

Technology, Cambridge, MA, 02139, USA

*Correspondence to email: jcg@mit.edu

ABSTRACT: Refractory Metals and their carbides possess extraordinary chemical and

temperature resilience and exceptional mechanical strength. Yet, they are notoriously difficult to

employ in additive manufacturing, due to the high temperatures needed for processing. State of

the art approaches to manufacture these materials generally require either a high-energy laser or

electron beam as well as ventilation to protect the metal powder from combustion. Here, we present

a versatile manufacturing process that utilizes tar as both a light absorber and antioxidant binder

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to sinter thin films of aluminum, copper, nickel, molybdenum, and tungsten powder using a low

power (< 2W) CO2 laser in air. Films of sintered Al/Cu/Ni metals have sheet resistances of ~10-1

ohm/sq, while laser sintered Mo/W-tar thin films form carbide phases. Several devices are

demonstrated, including laser sintered porous copper with a stable response to large strain (3.0)

after 150 cycles, and a laser processed Mo/MoC(1-x) filament that reaches T~1000 oC in open air at

12 V. These results show that tar mediated laser sintering represents a possible low-energy, cost-

effective route for engineering refractory materials, and one that can easily be extended to additive

manufacturing processes.

KEYWORDS: steam cracker tar, laser sintering, transition metal, refractory metal carbides, laser

manufacturing

Refractory metals such as molybdenum, tungsten, and their carbides, possess extraordinary

chemical and thermal resilience and mechanical strength.1,2 Tungsten and molybdenum carbide

has been widely applied in machine tools and armor-piercing shells,3 and their alloys have been

applied in parts of aeronautical engines including nozzles and turbine blades.4 However, they can

be difficult to engineer due to their high melting temperature.5–7 Unlocking low-cost processing

conditions for sintering and printing methods using refractory materials could lead to additive

manufacturing processes and applications, from biomedical engineering,8 to power plants,9 to

aerospace.10

Laser annealing has been shown to provide low-cost versatile thermal processing methods with

broad material compatibility and spatial resolution.11–14 In addition to controlling laser-specific

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parameters such as power, rastering speed, and focus, material synthesis paradigms can be

developed by exploiting light-matter interactions to enhance and localize thermal

transformations.15 State-of-the-art additive manufacturing methods such as selective laser melting

(SLM) and electron beam melting (EBM) require relatively high laser power (up to several kW)

while operating under vacuum or inert gas.16 Direct additive manufacturing (3D printing) of

ceramics and refractory, high entropy alloys is generally difficult due to their high melting

temperature and the hardly controllable phase transformation during the thermal processes.10,17

The current examples are still deployed by printing polymer based precursor followed by a post-

combustion process: silicon oxycarbide micro-lattice can be derived from preceramic polymers;

Al-Co-Cr-Fe-Ni alloy can be reduced by annealing in CO from 3D structure printed using polymer

blend with oxide nanopowder.7,18 These post-conversion processes induce volume shrinking,

porosity, and increase of impurities, and direct printing process will greatly help to resolve these

problems.

Herein, we present a versatile manufacturing process to sinter thin films of aluminum, copper,

nickel, molybdenum, and tungsten, using petroleum processing byproduct steam cracker tar (SCT)

as a binder.12 The adaptable viscosity of tar, combined with its enhanced light absorption properties

and flexible chemistry, facilitates the direct patterning and laser sintering of metal microparticles

into metal and metal carbide thin films in air without formation of oxides. Using tar as a light

absorber and antioxidant enables direct sintering in air with a low power (< 2W) CO2 laser, rather

than at the temperatures used in bulk annealing above melting temperatures (Table S1). Tar

displays an attenuation coefficient (~103 cm-1) two orders of magnitude lower than that of metals

(~105 cm-1) at the laser excitation wavelength (10.6µm),19–22 resulting in a deeper penetration depth

of the laser into the metal-tar thin film. Additionally, the lower attenuation coefficient of tar allows

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for higher efficiency in the photon to thermal energy conversion. By optimizing laser parameters

and metal powder size distribution, we show that laser-treated metal/tar thin films produce metal

phases (Al, Cu, Ni) and metal carbide phases (Mo, W) with low sheet resistance, as low as 10-1

ohm/sq. As proofs of concept, a printed copper auxetic pattern is transferred onto Ecoflex to

assemble a strain sensor with a gauge factor of ~20, and a printed molybdenum carbide thin film

joule heater can reach over 1000 oC in open air. Furthermore, we successfully demonstrate the tar-

mediated sequential vertical patterning of a double layer molybdenum carbide thin film, which

indicates the potential suitability of this method for additive manufacturing with refractory

carbides.

RESULTS AND DISCUSSION

Figure 1 ． Schematic of laser annealing of metal and refractory metal carbides using petroleum tar as the light-absorbing binder, at ambient conditions. (a) Attenuation coefficient of tar model calculated by first principles simulations. (b) Atomic model of tar. (c-d) Illustration of laser sintered metal thin film with tar binder. (e) Carbon segregation on metals with low carbon solubility like aluminum and copper after laser sintering. (f) Metals with high carbon solubility lead to a smooth surface after sintering; refractory metals molybdenum and tungsten form carbides.

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Steam cracker tar, the byproduct of ethylene cracking, is a combination of polycyclic aromatic

hydrocarbons (PAHs) and alkanes (Figure 1a-b). Tar absorbs around 10 µm due to the C-C

vibration in PAH.23 Using first principles simulations, the attenuation coefficient of tar at 10.6 µm

(~103 cm-1), as shown in Figure 1a-b (for the details in Figure S1 and Table S2) is found to be

much smaller than that of metals (~105 cm-1) (Table S1). Carbon materials such sparse vertical

aligned carbon nanotube forest, carbon black and graphitic could function as near blackbody

materials to increase the light absorptivity and emissivity.24 The heavy polyaromatics in tar are

transformed into a highly aromatic nano-scale framework during laser annealing, leading to the

formation of nano-scale cavities acting as black-bodies with near-unity absorptivity, 12 similarly

to nanoscale cavity absorption enhancements observed in carbon nanotubes.24 Such structural

property could enhance the laser absorptivity and emissivity within the tar-metal and lead to a

higher internal temperatures. A tar solution (15 wt %) is made by diluting as received tar by

dichloromethane (DCM). Thin films are synthesized by spin coating from a suspension of metal

powder (1~5µm particle size) in tar solution with mass ratio 100:15:85 (metal: tar: solvent)

(Figure 1c-d). The incident excitation laser incident power is found to be dissipated and reflected

internally among the metal particles, allowing it to penetrate deeper and increase the light/metal

power coupling.

As shown in Figure 1e-f and Figure 2, the structural conformation of sintered metal-tar thin

films depends on two factors: the carbon solid solubility in the given metal and the formation

energy of the corresponding metal carbide. In order to explore the roles of these two factors, five

metals of varying carbon solubility and carbide formation energies were chosen: Al, Cu, Ni, Mo

and W. Aluminum has both a relatively low melting temperature (~600 oC) and low solid carbon

solubility (0.015 wt %). Copper also possesses a very low carbon solid solubility,25 which has been

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exploited to control the growth of monolayer graphene by chemical vapor deposition.26 Ni has

high carbon solubility and strong chemisorption binding with carbon atoms.27,28 From the phase

diagram of Mo-C and W-C, the values of formation energy of their respective carbides are even

lower than melting Mo and W in the presence of carbon.29,30 (Figure 2j,m and Figure S2-4). Phase

segregation of carbon on the metal surface is not observed in laser annealed Ni-tar patterns from

SEM images, which can be attributed to the high carbon solid solubility of C in Ni. On the other

hand, a thin layer of amorphous carbon (<1/10 of film thickness) is always observed on the Cu-

based sintered film, as shown in the energy-dispersive X-ray spectroscopy (EDX) mapping and

Raman spectrum Figure S2. Similarly, a thin layer of carbon segregates to the surface of laser

sintered Al-tar thin films (Figure S3). We note that the intensity of the X-ray diffraction (XRD)

[2 0 0] reflection (52.4o, with a corresponding lattice distance of d=2.02A) increases with the

power/speed ratio indicating a recrystallization of Al powder in the laser sintering process. The

peak intensity ratio of Id=2.02 A/Id=2.34 A of Al and laser sintered Al using 1% rastering speed

(12.7mm/s) and 8% power (~2.4 W) increases from 0.61 to 5.25, indicating a partial (~40 %)

recrystallization of Fm3m (FCC) phase cubic to Im3m (BCC) cubic Al derived from XRD

(Figure S3b). We note that no crystallization is ever observed for Cu, possibly due to the lower

internal temperature resulting from a lower carbon content within the carbon-metal interface and

with the metal particle itself. With an increase in power/rastering speed ratios, the restructured Al,

Cu and Ni coalition increases after sintering, rather than changing the chemical composition (such

as forming a carbide phase or oxides in air, Figure S4). The carbon residual in sintered Al, Cu,

and Ni thin film should be close to the carbon solid solubility in those metals.

The XRD patterns shown in Figure 2j&m, highlight the formation of carbide phases in both

laser-sintered Mo-tar and W-tar films. Higher incident laser power density (power/speed ratio) not

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only increases the coalition size, but it also applies sufficiently high energy to induce the formation

of a mixed carbide/metal phase. Carbon in tar is over-saturated, which facilitates the formation of

metal carbide phases with either M2C or MC (1-x, 0≤x<0.5) stoichiometry. As shown in Figure 2j-o &

3 and Figure S5 & S6, with the increase of projected power density, the lower energy β-Mo2C

with lower formation temperature in the Mo-C phase diagram is observed as shown in the carbide

I regime. With further increase in incident power up to 2.4 W (at speed of 40 mm/s), a significant

amount (59 at%) of α-MoC(1-x) is observed as shown in the carbide II regime with 11 at% of

molybdenum metal. In the case of tungsten, lower energy phases δ-WC and α-WC(1-x) are observed

first. Using 12.7 mm/s as the fixed rastering speed, the β-W2C phase starts to occur in the sintered

metal thin film using 0.9 W (3% of power). With higher incident power (>1.5 W), all carbides

exhibit the β-W2C phase. With up to 2.1 W incident power, the sintered tungsten film has 62 at%

β-W2C phase with 38 at% metal tungsten. It is worth noting that the carbide phases α-MoC(1-x) and

β-W2C, which lie in the regime with higher metallurgical formation temperature, have higher

formation energy than the β-Mo2C phase and δ-WC and α-WC(1-x) phases.

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Figure 2．Laser sintered metal and refractory metal phases and microstructures. X-ray diffraction (XRD) pattern of laser-sintered metal-tar thin films and scanning electron micrograph (SEM) of as deposited and laser sintered metal-tar at different laser power and speed, for: (a-c) aluminum-tar; (d-f) copper-tar; (g-i) nickel-tar; (j-l) molybdenum-tar; (m-o) tungsten-tar. XRD in (a-c) shows the laser induced sintering leads to a preservation of the metal phases. XRD in (j) and (m) shows the laser sintering process leads to the formation of carbide phases. In (a,d,g,j,m) the first number after the “las-“label represents laser power and the second number represents speed in percentages. Scale bars: 10 µm.

Beside the crystallographic characterization of laser sintered metal/tar mixtures by XRD,

electrical conductivities of sintered films are used to validate the formation of either metallic or

carbide phases in refractory metals. Indeed, thin films show different trends in response to the ratio

of incident laser power/speed, as shown in Figure 3 and Figure S7. For aluminum, copper and

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nickel thin films with tar, conductivities are primarily determined by the geometry and

connectivity of a percolated single metal phase. The lowest sheet resistance, ~10-1 ohm/sq, can be

achieved by optimizing the laser power and rastering speed. As shown in Figure 3 and Figure S2-

7, metal-tar films are ablated by fixing either the laser power or rastering speed while varying the

other. We define the power/scan speed ratio as rastering power density. Figure 3d illustrates the

evolution of conductivities with power density. The evolution of metal phases and their

microstructures, as well as the electrical conductivity of the resulting thin films follows directly

from the incident rastering power density. High speeds and low power results in an “under

ablation” regime, while higher power densities result in the increase in interparticle connectivity

and conductivity of sintered metal patterns, leading to an optimal regime noted as “highest ” in

Figure 3c. Further increases in rastering power density into the “over ablation” regime led to

severe spheroidization of metal particles and a disconnected percolated network, due to the metal

evaporation under high projected power density (Figure S2-6). For the formation of carbide

phases, the relation between conductivity and power density is more nuanced. As shown in

Figure 3d, the regimes of “under ablation” and “over ablation” are characterized by a rastering

power density that is either too low to trigger sintering and recrystallization,31 or too high leading

to a disruption in the metal percolation network. In the case of Mo and W, the increase of laser

power and decrease of scan speed leads to formation of higher energy carbide phases (α-MoC(1-x)

and β-W2C phase) with interparticle connectivity and thus increased conductivity (Figure 3d,

Figure S5-6). The high conductivity regime is carbide II, and the conductivity increases while

transitioning from carbide phase I to II. In the case of Mo, I-II refers to the β-Mo2C to α-MoC(1-x)

transition. In the case of W, I-II refers to the α-WC(1-x) to β-W2C transition.

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Figure 3. Electrical conductivities () of laser annealed metal-tar thin films of (a) Cu-tar; (b) Ni-tar; Qualitative depiction of the evolution of: (c) non-carbide metal-tar thin films and (d) formation of carbide phases in refractory metal-tar thin films controlled by laser power and speed. Rastering speed (%) refers to the percentage to the maximum value (1270 mm/s), power (%) refers to the percentage of the full power (30W).

The initial size of metal powder in the metal-tar thin film is critical in determining the

morphology and conductivity of the as-sintered metal thin film. As shown in Figure S8a-c, Mo-

tar thin films are made from tar-metal solution with mass ratio 100:15:85 (metal: tar: solvent),

using smaller powder size (~100 nm) following the same deposition and lasing processes for

micro-size particles. Those thin films result in much higher sheet resistance (102 ohm/sq), while

the lased thin film presents significantly deeper microfractures than laser-ablated Mo-tar using

micron size powder (Figure S8d-f). The fractures can be attributed to the larger dewet induced

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deformation and shrinking due to merging of particles in the laser sintered metal film, which could

be improved by increasing the packing density of metal powder in the tar-metal thin film. To

further increase the packing volume fraction32, we introduced metal powder with a smaller size

(~100nm) into the 1~5 µm size molybdenum powder with a mass ratio of 1:1 to make a Mo(mix)-

tar solution with the same mass ratio of tar and solvent in mono-size tar-metal solutions. Laser

sintered Mo(mix)-tar showed clear improvement compared to sintered Mo-tar using mono-

distributed micron size particles, displaying fewer visible cracks, and slightly lower sheet

resistance – decreased from 4.1 ohm/sq to 3.0 ohm/sq (Figure S8g-i).

We show next that devices can be fabricated through patterning using the optimized laser

parameters that give the highest conductivity (supplementary notes). Laser annealing with high

spatial resolution can be used to tailor the device geometry on temperature-sensitive substrates

such as polyimide. Residual un-lased copper-tar is washed off using dichloromethane (DCM)

(Figure 4a). The laser sintered copper pattern can be transferred onto flexible substrates such as

Ecoflex to fabricate stretchable electronic devices, in this case a strain sensor (Figure 4b). The

resistance of the as-printed auxetic pattern shows a linear response with applied strain up to 0.15,

with a gauge factor ~20, which is much higher than the elasticity limit of copper metal.33 The

auxetic pattern and the interconnected porous copper microstructures enables the repetitive

application of large strains beyond 150 cycles (Figure S9). Laser annealing a Mo/MoCx in a wire

form-factor, shows low resistivity down to 0.5 ohm/sq, making it an ideal candidate for a Joule

heating filament.34 Figure 4c shows that in such an application, the filament can reach up to ~1000

oC in open air with an input voltage of 12 V.

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Figure 4. Application of laser annealed metal-tar microparticles for thin film electronics. (a) A pattern is printed on as-deposited Cu-tar film and transferred onto Ecoflex. (b) A strain sensor of auxetic copper pattern sandwiched between two layers of Ecoflex. (c) Maximum temperature values of Mo/MoCx joule heating filament working in air corresponding as a function of input voltage. Inset, optical image of a joule heater printed on a quartz substrate with 0.5 inch diameter, with the temperature profile powered by a 12 V input voltage. The temperature color map was captured by the Infrared camera (FLIR). (d) Schematic of double layer metal printing using tar as absorption binder. (e) SEM image of printed double layer molybdenum carbide, with higher resolution area shown in (f) highlighting the bonding interface between two vertical layers. Scale bar in (e) and (f): 50 μm and 20 μm.

The viability of the laser sintering process in additive manufacturing is tested by synthesizing a

vertical sintered double layer. Here, a layer-by-layer coating and printing is achieved through

repeated sequences of depositions, pattering, laser annealing and residual washing off. While

residual carbon on the laser sintered Cu and Al thin films will block the formation of interlayer

bonding, carbide formation in Mo-tar and W-tar could potentially bond the two layers. As a proof-

of-concept, a double layer of a Mo/MoCx pattern is demonstrated in Figure 4d-f. After removing

the residual powder, the bonding between the two layers of Mo/MoCx can be seen from the SEM

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of Figure 4f. The first layer molybdenum sintering is performed using low power (1.8 W) and low

rastering speed (12.7 mm/s) to increase the packing density of the sintered structure. The second

layer is sintered with relatively high power (~4.5 W) and a rastering speed of (101.6 mm/s) to

increase the penetration depth and interlayer bonding. Such a result indicates the potential of

extending the tar mediated laser sintering of metal carbide formation into additive printing

processes.

CONCLUSION

In summary, we present tar-mediated laser sintering of metal nano- and microparticles including

Al, Cu, Ni, and refractory Mo and W using relatively low power (< 2 W) under ambient conditions.

A thin layer of carbon segregates on aluminum and copper sintered films surface due to its low

solubility in these metals,25 while nickel with higher carbon solubility provides a clean surface

after laser annealing.28 Refractory metals (molybdenum and tungsten) which are laser sintered in

air, result in different high energy transition metal carbide phases,29,30 through optimized control

over laser power and rastering speed. By exploiting the microstructure and high electrical

conductivity of the laser sintered metal and metal carbide thin films, we demonstrate the successful

fabrication of a strain sensor with a laser-printed copper auxetic pattern with linearity behavior up

to 15% strain, and a gauge factor of ~20. Laser printed Mo/MoCx Joule heating filaments are also

fabricated with a sheet resistance below 1.6 ohm/sq, leading to an operational maximum

temperature over 1000 oC in the open air with an input power of 12 V. Furthermore, we

demonstrated a proof-of-concept sequential multi-layer printing process of molybdenum carbide,

with a method that can be potentially expanded and integrated into additive manufacturing

processes.

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Most importantly, with the assistance of an absorbing binder the input energy is two orders of

magnitude smaller than what is required in other metal printing methods such as selective laser

melting (SLM),4,35,36 and electron beam melting (EBM).37,38 Without using tar in the metal thin

film, the low power down to 1W cannot trigger any photon-induced thermal reactions.

Furthermore, the laser-induced metal sintering method here proposed operates under ambient

conditions, eliminating the need for ventilation and pumping, in turn decreasing the processing

time and complexity compared to SLM and EBL. Direct laser printing in air through a compact

laser source reduces the processing overhead imposed by the otherwise required control over the

buffer gas during annealing and instrumentational complexity in the case of EBM. Such factors

could potentially reduce the complexity of fabricating metal and metal carbide films in continuous

“roll-to-roll” manufacturing. Furthermore, the tar-metal ink could potentially be adapted to an

extrusion-based 3D printer with fiber optics, and thus be done in an additive way of printing.

MATERIALS AND METHODS

Materials

Stream cracked tar was provided by ExxonMobill (Defluxed SOP2 Tar from 15-082837).

Aluminum powder (1~5 µm) and copper powder (<45µm) were purchased from Sigma Aldrich.

niickel powder (3~7 µm), molybdenum powder (2~4 µm), and tungsten powder (1~5 µm) were

all purchased from Alfa Alsar. Dichloromethane (DCM, CAS 75-09-2) was purchased from Sigma

Aldrich.

Materials Analysis and Characterization

A profilometer (Bruker DXT Stylus Profilometer) was used to map the thickness of spin coated

and laser ablated thin films. Scanning electron microscopy (SEM, Hitachi SU8100) was used to

image the laser sintered metal-tar thin films. Energy-dispersive X-ray spectroscopy (EDX)

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mapping was taken in SEM using an Oxford EDX detector (15 kV, 20 μm) controlled by an APEX

software interface. Micro-Raman spectra were acquired using a Renishaw inVia™ confocal

Raman microscope using a 473 nm excitation source. The laser spot on the sample was ~800 nm

in diameter and had a power of ~4 mW at the sample surface. The full spectral window for each

acquisition is from 600 to 3200 cm-1. X-ray diffraction patterns of metal-tar thin films were tested

using the Bruker D8 Discovery, and semi-quantitative analysis of patterns was performed using

HighScore software to identify the phases and estimate the components.

Laser Sintering of Metal-tar Thin Film

A tar solution (15 wt%) was made by diluting as received tar by dichloromethane (DCM). Thin

films were created by spin coating a suspension of metal powder in the tar solution with mass ratio

100:15:85 (metal: tar: solvent). Metal powders and their physical properties are summarized in

tab. S1. The tar solution was spun coated onto glass substrates at a speed of 500 rpm for 20 seconds,

and the deposited metal-tar thin films were dried in air for 4 hours and heated up to 80 oC to remove

the volatile component and residual solvent. A commercial laser cutter (Universal VSL 2.30) was

utilized to perform the laser annealing and sintering, with a CO2 laser tube (maximum power of

30W) and all built-in optics. The laser spot was focused on the top surface of the spin-coated films

by tuning the z-position (height) of the supporting cutting table. The designed pattern was imported

to a vector graphics software (Inkscape) and engraved by communicating with the laser cutter as

a printer. On different metal-tar thin films, different parameters of power and speed were crossed

compared for maximum conductivity at focus.

Prototyping Laser Sintered Metal Devices

To prototype more complex designs for metal devices, the laser was calibrated to the parameters

that achieved the highest conductivity. Using a computer aid design (CAD) interface, many

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features were directly printed on tar-metal film. As-ablated devices were rinsed in solvent DCM

to remove the unablated areas, and air dried followed with heat up to 100 oC to fully remove

solvent. Copper-tar printed in the patterns in Figure 4a are sintered by laser annealing and washed

with solvent to remove residual copper powder and transferred onto Ecoflex to make a stretchable

strain senor.

Conductivity Measurement

A lakeshore TTP4 probe station was used for sheet resistance characterization. Parameter

analyzer (4156C) is interfaced with a PC running EasyExpert software to setup any test with up to

1mA and 5V. All room temperature electrical characterization was done under air.

Testing of the Mo/MoC1-x filament

Mo-tar thin films were deposited onto quartz substrates of 0.5-inch diameter. Laser sintering

was performed as described above with the calibrated power and speed parameters with the highest

resulting conductivity. Two electrodes were deposited using silver paste on the edge of the

Mo/MoCx thin film. The heating performance of the filament was tested using a DC voltage source,

ranging from 1-12 V with a step of 1 V. At each voltage input, the temperature was recorded by

an IR camera (FLIR A 615) for 10 min until the temperature saturated.

ASSOCIATE CONTENT

Supporting Information

The Supporting Information is available free of charge at xxxx;

The Supplementary materials includes：

Physical and optical properties of tar and metals tested, simulation set up of tar, SEMs of laser sintered metal-tar thin films, conductivities () of laser annealed metal-tar thin films, and a demonstration of strain sensor made of laser sintered copper.

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AUTHOR INFORMATION

Corresponding Author

* Jeffrey. C. Grossman, email: jcg@mit.edu

Author Contributions

X.Z. and J.C.G. designed the research; X.Z. designed and performed experiments; K.Y.T. helped

with metal sintering; C. J. performed the simulation; W.S. helped with the test of printed sensors;

N.F. provided insight on tar chemistry, film and process characterization; X.Z., N.F. and J.C.G.

wrote the paper; all the authors provided comments and feedback on the manuscript.

ACKNOWLEDGMENT

The Tar used in this work was provided by ExxonMobil. All authors claim no competing interests.

This work made use of the MRSEC Shared Experimental Facilities at MIT, supported by the

National Science Foundation under award number DMR-14-19807.

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